A Study on the Influence of Radial Spoiler Arrangement on the Combustion Process of Wankel Rotor Engines

Abstract

Wankel rotor engines are widely used in various fields due to their high power density and simple structure. This paper presents the optimisation of the Wankel rotor engine by simple modifications of the structure. We propose a radial spoiler arrangement scheme that can affect the flame propagation speed and reaction severity by altering the flow field distribution and pre-reaction distribution in the cylinder. By comparing the effects of four layout schemes on flame propagation speed and reaction intensity, including no spoilers, radial spoilers deflected at an angle of 10° in the negative direction of the Z-axis, radial spoilers deflected at an angle of 10° in the positive direction of the Z-axis, and radial spoilers arranged at the centre of the rotor combustion chamber, the benefits of different layout schemes were evaluated. We conducted a study on the influence of the arrangement of radial spoilers on the combustion process of a Wankel rotary engine through theoretical calculations. This helps to reduce engine vibration, improve engine operation stability, and enhance engine performance.

1. Introduction

Rotary engines face challenges such as high fuel consumption and significant pollutant emissions. Nonetheless, their compact structure, low number of required parts, excellent high-speed performance, and high power output render them suitable for specialised applications. Consequently, they have swiftly found practical uses across various sectors since their inception. With advantages of robust power performance, adaptable fuel usage, high power-to-weight ratio, small size, and light weight, they are widely employed in passenger vehicles, portable engine sets, special vehicles, unmanned aerial vehicles, and small aircrafts, with particular popularity in the military field. For instance, rotary engines are used in special equipment, like tracked vehicles and high-speed boats, and serve as the primary propulsion system for reconnaissance drones and some small manned aircrafts. Rotary engines have promising potential for development, including new application prospects in the future. Like conventional engines, both the combustion chamber [1,2] and turbulence conditions within the rotary engine profoundly influence the combustion and performance of the engine. Meeting evolving demands necessitates enhancing the economic efficiency of rotary engines through various means.

Regarding improving the performance of rotary engines, the focus is on improving the design of the combustion chamber. Enhancing the combustion chamber leads to improved combustion conditions and enhances engine thermal efficiency. Since the advent of the rotary engine, researchers have refined the combustion chamber to bolster engine performance. For example, Kuo et al. [3] used numerical analyses to study the effect of the rotor profile on engine compression flow. Their study showed that increasing the shape coefficient K enhances compression efficiency and mixture formation. Various scholars have explored the impact of the shape of the combustion chamber groove on the engine’s internal flow field [4], indicating that different groove sizes affect fuel–air mixing. The degree of mixing of fuel and air is influenced by different groove sizes. Reducing groove sizes heightens pressure within the combustion chamber, thereby boosting the engine output power [5]. Additionally, groove placement affects engine flame propagation and development [6,7].

In addition to refining the grooves of the combustion chamber, integrating spoilers into the chamber is another method to optimise the combustion chamber and improve combustion performance. Numerous studies have shown that placing spoilers in the combustion chamber significantly improves combustion flow performance. For example, Peng et al. [8] investigated the effects of spoiler placement on the combustion process and found that increased proximity to the spark plug increases the turbulence velocity and dissipation rate in the spark area. In rotor combustion chambers with guide vanes, faster mixture combustion occurs, leading to improved combustion characteristics and emission performance. Zhu et al. [9] conducted numerical analyses on the total pressure loss, combustion efficiency, speed, and temperature under different spoiler structural parameters, concluding that built-in spoilers stabilise combustion, promote oil and gas mixing, improve combustion efficiency and temperature distribution, and reduce pollutant emissions [10,11,12,13].

Based on the research findings of the aforementioned scholars, it is evident that optimising the rotary engine’s combustion chamber significantly impacts combustion performance, particularly with the incorporation of spoilers in the rotary engine’s combustion chamber. Therefore, based on a three-dimensional simulation model, this study establishes different arrangements of spoilers to investigate the influence of combustion chamber spoilers on the combustion performance of rotary engines. In contrast to conventional axial placement, this study focuses on radially arranged spoilers. By designing spoilers at three different radial angles and comparing them with chambers lacking spoilers, this study analyses the influence of radial spoilers on combustion and compares the differences in combustion performance under different spoiler arrangement angles to ascertain optimal arrangement schemes [14,15,16]. The research herein offers guidance for the optimal design of the rotary engine combustion chamber and the optimal arrangement of the guide vanes. Furthermore, investigating the influence of spoilers on the in-cylinder flow field and combustion at different angles provides technical and theoretical support for improving the performance of rotary engines.

Unlike traditional engines that convert the reciprocating motion of the piston into the rotational motion of the crankshaft, the Wankel rotor engine uses the expansion work generated by the combustion of the combustible mixture in the combustion chamber to rotate the Wankel rotor, which then acts on the journal of the eccentric shaft, generating tangential force. With the support of the front and rear main bearings, the main shaft rotates and does work, producing mechanical energy. Similarly, the working process of a delta rotor engine can also be divided into four strokes: intake, compression, power generation, and exhaust. Its comparison with traditional piston engines is shown in Figure 1. The interactions between the internal and external gears within the Wankel rotor engine results in a working cycle of 1080°, while also possessing certain characteristics of a two-stroke engine [17,18,19].

Establishment of Simulation Model of Wankel Rotor Engine

This study used CONVERGE 3.0 to build a three-dimensional simulation model. The software operates within the Windows 10 environment, and the workstation configuration includes the following: Intel C621 series chipset; Intel Xeon Gold 6145 processor, featuring 40 cores and 80 threads, with a main frequency of 2.0 GHz, and a maximum turbo frequency of 3.7 GHz; 128 GB of memory; and NVIDIA GT1030, with a 2 GB graphics card. The focus of this study is on a single-cylinder gasoline rotary engine for research and validation purposes. The structural parameters of this rotary engine are shown in

Table 1. Structural parameters of rotary engine.

Parameter	Numerical Value
Generating radius (mm)	76
Eccentricity (mm)	12
Rotor width (mm)	50
Translational distance (mm)	1.45
Rated speed (r/min)	8000
Single cylinder volume (cc)	131
Compression ratio	11
Ignition timing	30°CA
Ignition source	Single spark plug

. The solid model and computational fluid domain model established based on these parameters are shown in Figure 2.

This article uses the RNG K–ε model (Renormalisation K–ε Group Model) to calculate the airflow motion inside the cylinder and combines the PRF (Primary Reference Fuel) skeleton mechanism, containing 41 components and 124 chemical reactions, with SAGE (Software for the Analysis of Gas phase chemistry and Aerosol formation in the Environment, v. 50). This computer program has been developed for studying atmospheric chemistry and aerosol formation through combustion calculations using a detailed chemical kinetics model. A three-dimensional simulation model of a rotary engine was established to investigate the flow and combustion processes inside the cylinder at different heights of the diffuser plate. Since the number of grids can impact calculation results, it is necessary to perform a grid independence analysis on the simulation model to ensure calculation accuracy and select the most appropriate number of grids to minimise calculation time. This study compares in-cylinder pressure changes at four different mesh numbers while keeping the boundary conditions constant. The boundary condition settings of the rotary engine are shown in

Table 2. Boundary conditions of rotary engines.

Border Area	Type	Temperature (K)	Pressure (Par)
Inlet	inflow	300	101,325.0
Intake port	Fixed wall	300	/
Outlet	outflow	570	101,325.0
Exhaust port	Fixed wall	550	/
Rotor	Moving wall	400	/
Rotor Flank1	Fixed wall	630	1,171,325.0
Rotor Flank 2	Fixed wall	350	101,190.0
Rotor Flank 3	Fixed wall	560	102,200.0
Spark Plug	Fixed wall	624.05	1,172,100
Spark electrode	Fixed wall	600	/

. Figure 3 shows the pressure change curves at different mesh numbers. Adaptive mesh refinement was employed based on a mesh number of 2 mm. The cylinder pressure change curve closely aligns with that of a mesh number of 1 mm, indicating stability in our calculation results [20,21,22]. Hence, this study opted for 2 mm adaptive meshing for calculations, considering efficiency, accuracy, and calculation time comprehensively. The total number of grids used in the calculation process for the Wankel rotor engine in this study was approximately 20 million hexahedral unit grids, and their distribution is shown in Figure 4.

Unlike common smooth rotor combustion chambers or those with axial turbulence spoilers, this study investigates the impact of radially arranged spoilers on the combustion process. Like axial turbulence spoilers, radially arranged spoilers offer flexible placement, minimal modification to the rotor structure, and minimal impact on the compression ratio [23,24,25]. To fully study the impact of radial spoilers on the combustion process, flame propagation, and performance, this study set up three different radial spoiler layout schemes alongside a smooth rotor scheme without spoilers for comparative analysis. Figure 5 depicts the structural schematic diagrams of the four rotor combustion chamber schemes. These schemes are denoted as Case0 through Case3, respectively. Case0 features a combustion chamber without spoilers, while the radial spoilers in Case1 are deflected by 10° in the negative direction of the Z-axis. In Case2, radial spoilers are deflected by 10° in the positive direction of the Z-axis, while Case3 positions the radial spoiler at the centre of the rotor combustion chamber.

2. Methods and Results

2.1. Analysis of the Influence of Radial Spoilers on Flame Propagation Speed

To accurately assess the consistency of flame propagation velocity in different combustion chamber structures along the Z-axis direction, this study divided the combustion chamber of the rotary engine into two equal parts that are perpendicular to the Z-axis (the positive and the negative direction of the Z-axis, respectively), with the centre as the midpoint, as shown in Figure 6.

To study the effect of radial spoilers on flame propagation speed, the positions of flame fronts for different combustion chamber structures were plotted at 0°CA TDC (Crankshaft Angle Top Dead Centre), 30°CA TDC, and 60°CA TDC, at a temperature of 2000 K, on the Y–Z section at the centre of the spark plug. The flame propagation velocity was then calculated in both the positive and negative directions. The calculation involved measuring the change in the flame front position at a temperature of 2000 K and converting it to the relative rotor linear velocity. The flame front position at 2000 K indicated the primary combustion area, while the flame propagation speed assessed the consistency of the flame propagation in the positive and negative directions. Figure 7 shows the flame front positions under different spoiler arrangements, while Figure 8 shows the change curve of flame propagation speed in both the positive and negative directions with the crankshaft angle (CA) for the four combustion chamber layout schemes. It is evident from the figures that the arrangement of the radial spoilers has a great impact on the consistency of the flame propagation velocity. The flame propagation velocity in the positive direction of the Case0 scheme without radial spoilers was significantly greater than the velocity that was observed in the negative direction. The peak flame propagation velocity in the negative direction was less than the velocity that was observed in the positive direction, with the positive direction reaching a peak relative velocity of 5 near 0°, while the negative direction reached a peak relative velocity of 2.5 near 25°, representing the worst consistency among all of the observed measurements of flame propagation velocity. Upon the addition of radial spoilers, the peak values of the flame propagation speed in both the positive and negative directions occurred roughly simultaneously. However, the consistency of the flame propagation speed varied. For instance, in the Case1 scheme, with radial spoilers deflected by 10° in the negative direction of the Z-axis, the flame propagation speed in the negative direction exceeded that in the positive direction, with relative velocity peaks of 4.2 and 5.5, respectively. Conversely, in the Case2 scheme, with radial spoilers deflected by 10° in the positive direction of the Z-axis, the flame propagation speed in the positive direction surpassed that in the negative direction, with relative velocity peaks of 7.3 and 4.9, respectively. In the Case3 scheme, where the radial spoiler was arranged at the centre of the rotor combustion chamber, the flame propagation velocities in both the positive and negative directions were nearly identical, with relative velocity peaks of five each. Generally, the consistency of the flame propagation speed was highest in the Case3 scheme, followed by Case1. Case2 exhibited the poorest consistency among all the schemes with radial spoilers, although it still outperformed Case0 (without radial spoilers).

Figure 9 shows the in-cylinder flow field of Case0 at −30°CA TDC. It can be seen from the figure that, due to the angle of the inlet arrangement, the vortex generated by the air flowing into the cylinder results in an asymmetrical flow field within the cylinder, which contributes directly to the poor consistency of the flame propagation speed. To allow for further analysis of the velocity distribution in the cylinder, Figure 10 presents the velocity distribution in the Z-axis direction on the Y–Z plane at the centre of the spark plug for the different combustion chamber structure schemes at 0°CA TDC, 30°CA TDC, and 60°CA TDC. It can be seen from the figure that the arrangement of radial spoilers has a great influence on the velocity distribution within the cylinder. The velocity distribution in the positive direction of the Case0 scheme exceeded that in the negative direction, which directly correlated with the higher flame propagation speed observed in the positive direction. However, upon introducing different radial spoilers, different velocity distribution results were observed. In Case1, the velocity distribution in the negative direction was greater than that in the positive direction. Conversely, the velocity distribution in the positive direction of the Case2 scheme was greater than that in the negative direction. Meanwhile, the velocity distributions in both the positive and negative directions of the Case3 scheme were nearly identical. Generally, the speed consistency was highest in the Case3 scheme, followed by Case1. Case2 exhibited the poorest consistency among all solutions with radial spoilers, although it still outperformed Case0 (without radial spoilers). The velocity distributions in the positive and negative directions of the Z-axis closely align with the flame propagation velocity distributions analysed in the preceding sections. Therefore, this study posits that radial spoilers impact flame propagation velocity by altering the flow field distribution within the cylinder.

2.2. Analysis of the Impact of Radial Spoilers on the Severity of the Reaction

To investigate the intensity of the combustion reaction, this study employed the reaction intermediate product CH₂O as an indicator to evaluate the distribution and intensity of the reaction. The distribution of the intermediate product CH₂O was plotted at 0°CA TDC, 30°CA TDC, and 60°CA TDC for the different combustion chamber structure schemes on the Y–Z section at the centre of the spark plug. Additionally, the severity of the reaction was calculated by normalising the average concentration of the intermediate product. Figure 11 shows the distribution of CH₂O under the different spoiler layout schemes, while Figure 12 shows the curves of the reaction intensity in the positive and negative directions of the four combustion chamber layout schemes as a function of the CA. The arrangement of the radial spoilers had a huge impact on the severity of the reaction. In the Case0 scheme (without radial spoilers), CH₂O appeared in the positive direction before appearing in the negative direction near 0°CA TDC. The distribution of the intermediate product CH₂O was quite different from the position of the flame front at a temperature of 2000 K. Conversely, in Case1, CH₂O appeared in the negative direction before appearing in the positive direction. Although the distribution consistency of CH₂O in the positive and negative directions near 30°CA TDC was poor in Case2, it was roughly consistent with the position of the flame front at a temperature of 2000 K. However, in Case3, the distribution consistency of CH₂O was the best from 0°CA TDC to 60°CA TDC. This indicates that the combustion intermediate product CH₂O is most consistent with the flame front position at a temperature of 2000 K.

Figure 13 shows the temperature distribution of different combustion chamber structure schemes at 0°CA TDC, 30°CA TDC, and 60°CA TDC on the Y–Z section at the centre of the spark plug. A comparison between Figure 11 and Figure 13 reveals that the abnormal distribution of the intermediate product CH₂O when the Case0 and Case1 schemes are near 0°CA TDC is not caused by combustion but rather by a precursor reaction of combustion. The reaction process, as shown in Formula (1), decomposed the fuel into CH₂O while releasing less heat. Combining this with the preceding analysis, it became evident that the arrangement of different radial spoilers impacted both the location and intensity of the front reaction. The consistency of the front reaction distribution of the Case0 scheme (without radial spoilers) was poor. The reaction distribution was more concentrated in the positive direction near 0°CA TDC, resulting in a more intense subsequent combustion reaction in that direction compared to the negative direction. Conversely, in Case1, the front reaction was more concentrated in the negative direction near 0°CA TDC, leading to a more intense subsequent combustion reaction in that direction compared to the positive direction. In contrast, the Case3 solution had a more consistent front reaction in both the positive and negative directions from 0°CA TDC to 60°CA TDC. This consistency resulted in a more uniform subsequent combustion in both directions. These findings align closely with the previous results on reactant intermediate products. Therefore, this study asserts that radial spoilers affect the distribution and intensity of the entire reaction by affecting the distribution and intensity of the precursor reaction.

$${IC}_8{H}_{18}+H{O}_2\leftrightarrow {IC}_8{H}_{17}+H_2{O}_2$$

$$\begin{gathered} {\mathrm{C}}_8{\mathrm{H}}_{17}={\mathrm{C}}_3{\mathrm{H}}_7+{\mathrm{C}}_3{\mathrm{H}}_6+{\mathrm{C}}_2{\mathrm{H}}_4 \\ {\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{M}=\mathrm{O}\mathrm{H}+\mathrm{O}\mathrm{H}+\mathrm{M} \\ {C}_3{H}_4+OH=C_2{H}_3+C{H}_{2}O \end{gathered}$$

(1)

2.3. Analysis of the Impact of Radial Spoilers on Combustion Consistency and Performance

Figure 14 shows the combustion intensity of the different radial spoiler layout schemes from −30°CA TDC to 180°CA TDC. Figure 15 shows the distribution and consistency of combustion intensity with the change of CA for different radial spoiler layout schemes from −30°CA TDC to 180°CA TDC. The consistency of combustion intensity was obtained by calculating the difference in reaction intensity at equidistant positions from the centre plane, perpendicular to the centre plane, in both the positive and negative directions. The smaller the consistency index of combustion intensity, the better the consistency of the reaction and the more similar the intensity of the reactions on both sides, with the central plane serving as the symmetry axis. It can be observed from the figure that, during the entire combustion process, the Case3 scheme has the best combustion consistency. In particular, Case3 maintains particularly an excellent combustion consistency from 0°CA TDC to 60°CA TDC. The Case2 scheme exhibited the second-best combustion consistency, although it exhibited the worst consistency near 0°CA TDC. The combustion consistency of the Case1 scheme showcased the worst combustion consistency among all schemes with radial spoilers; albeit, this was still better than the Case0 scheme (without radial spoilers). The poorest combustion consistency occurred around 30°CA TDC. As the combustion consistency improved, the peak of combustion intensity was delayed. Compared with the Case0 scheme, the Case1 scheme demonstrated an improvement of 30.5% in combustion consistency, the Case2 scheme showed an improvement of 15.7%, and the Case3 scheme exhibited a remarkable enhancement of 52.8%.

Figure 16 shows a comparison of cylinder temperature, cylinder pressure, and heat release rate for the different radial spoiler layout schemes. It is apparent that the radial spoiler arrangement significantly influences performance. The presence of radial spoilers delayed the maximum values of cylinder temperature and heat release rate by approximately 20°CA. Different radial spoiler arrangements also yielded different effects on cylinder pressure. The explosion pressure of the Case0 scheme was 3.06 MPa, while the explosion pressure of the Case2 scheme was 3.24 MPa, indicating a 5.8% increase compared to Case0. The explosion pressure of Case3 was 3.14 MPa, which was 2.75% higher than that of Case0. The explosion pressure of the Case1 scheme was 2.98 MPa, representing a 2.7% reduction compared to Case0.

The thermal efficiency and pollutant emissions of different radial baffle layout schemes are shown in Figure 17. From the figure, it can be seen that the arrangement of radial spoilers has a significant impact on performance and emissions. The thermal efficiency of the Case0 scheme without a radial spoiler is 32.40%. Compared to this, the thermal efficiency of the Case2 scheme, with a radial spoiler arranged at 10° in the positive Z-axis direction, is 35.67%, representing an increase of 10.08%. The thermal efficiency of the Case3 scheme, with radial baffles arranged at the centre of the rotor combustion chamber, is 34.53%, representing an increase of 6.56%. The thermal efficiency of the Case1 scheme, which arranges radial spoilers with a 10° deviation towards the negative Z-axis direction, is 30.65%, representing a decrease of 5.41%. With respect to the emission of HC pollutants, the emissions of different radial baffle arrangements are similar, ranging from 9.63 × 10⁻⁶ kg to 9.75 × 10⁻⁶ kg. However, different radial baffle arrangements have a significant impact on the emission of NOX pollutants. The NOX pollutant emissions of the Case0 scheme, without a radial spoiler, are 1.55 × 10⁻⁷ kg. Compared with this, the NOX pollutant emissions of the Case2 scheme, with a radial spoiler arranged at an angle of 10° in the positive Z-axis direction, are 1.12 × 10⁻⁷ kg, representing a decrease of 27.88%. The NOX pollutant emissions of the Case3 scheme, with radial baffles arranged at the centre of the rotor combustion chamber, are 1.25 × 10⁻⁷ kg, representing a reduction of 19.23%. The NOX pollutant emissions of the Case1 scheme, with radial spoilers arranged 10° in the negative Z-axis direction, are 1.78 × 10⁻⁷ kg, representing an increase of 14.74%.

3. Conclusions

Based on the in-cylinder combustion and flow characteristics of the Wankel rotor engine, this study proposes a radial spoiler arrangement scheme that can affect the flame propagation speed and reaction severity by altering the flow field distribution and pre-reaction distribution in the cylinder. By comparing the effects of four layout schemes on flame propagation speed and reaction intensity, including no spoiler, radial spoilers deflected at an angle of 10° in the negative direction of the Z-axis, radial spoilers deflected at an angle of 10° in the positive direction of the Z-axis, and radial spoilers arranged at the centre of the rotor combustion chamber, the benefits of the different layout schemes were evaluated, and the following conclusions were obtained:

(1). Flame propagation speed: The presence and arrangement of radial spoilers affects the flame propagation speed. Spoilers can synchronise peak flame propagation speeds in both the positive and negative directions, with the arrangements influencing speed consistency. Specifically, spoilers angled in different directions impact the relative speeds in each direction, with the centre-aligned spoiler resulting in almost identical speeds.

(2). Flow field consistency: The asymmetry in the cylinder’s flow field, caused by the inlet arrangement of the engine, leads to poor consistency, primarily affecting flame propagation speed. However, radial spoilers significantly enhance flow field consistency, particularly when positioned at the centre of the combustion chamber.

(3). Reaction intensity: Radial spoilers play a crucial role in determining reaction intensity. Different arrangements affect the distribution and consistency of reaction intermediates, with arrangements featuring spoilers at the chamber’s centre exhibiting the best consistency across different CAs.

(4). Pre-reaction impact: Abnormal distribution areas of intermediate products are attributed to pre-reactions, influenced by the arrangement of radial spoilers. Spoiler placement significantly affects the location and intensity of pre-reactions, with centre-aligned spoilers promoting consistency in subsequent combustion reactions.

(5). Overall performance: Radial spoilers have varying effects on performance and combustion consistency. Centre-aligned spoilers demonstrate the best combustion consistency, with a notable improvement in explosion pressure. Spoilers angled in different directions offer performance improvements but may vary in their impact on combustion consistency and explosion pressure.

(6). Reasonably arranged radial spoilers can effectively reduce NOX emissions. Compared with the scheme without radial spoilers, the scheme with radial spoilers arranged at an angle of 10° in the positive Z-axis direction has the largest reduction in NO_X pollutant emissions, which is 27.88%.

This study proposes a new direction for the performance optimisation of the Wankel rotor engine and confirms that rationally arranged radial spoilers can simultaneously improve response consistency and performance. Future research should focus on theoretical analysis and layout optimisation, integrating various spoilers and combustion chamber structures to further optimise the performance of the rotary engine.